Exterior-coated nitrile rubber article having natural rubber characteristics

ABSTRACT

A material for making elastomeric nitrile rubber articles is disclosed. In particular, the process and material formulation can produce nitrile rubber based articles, which exhibit force-strain characteristics that are comparable to those of natural rubber latex articles, while maintaining the tensile strength properties of nitrile rubber. The process includes an accelerator composition at the pre-cure stage having a dithiocarbamate, a thiazole, and a guanidine compound. The invention also includes an elastomeric nitrile rubber product made by the process, such as examination, surgical, or work gloves, which can have an exterior coating over naked rubber selected from either a silicone emulsion or oil, glycerin solution, a combined mixture of silicone and glycerin/glycerol, a fluorocarbon release agent, or a natural or synthetic hydrocarbon-based oil in amounts that do not degrade said substrate rubber material.

This application claims the benefit of priority from U.S. ProvisionalApplication No. 60/781,017 filed on Mar. 10, 2006 and U.S. ProvisionalApplication No. 60/680,971 filed on May 13, 2005 and U.S. patentapplication Ser. No. 11/195,030 filed on Aug. 2, 2005, the contents ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to elastomeric articles that are made fromnitrile rubber formulations. In particular, the invention pertains tocarboxylated acrylonitrile butadiene rubber articles that exhibitphysical characteristics that are comparable to similar articles madefrom natural latex rubber.

BACKGROUND OF THE INVENTION

The development of modern rubber materials has made possible themanufacture of a wide range of elastomeric articles having varyingproperties of strength and chemical resistance. As synthetic latexmaterials have developed, various elastic and polymeric materials havebeen adapted for use in making a variety of articles of manufacture. Oneuseful class of synthetic rubber material compounds includes the nitrilerubber class, which is widely used to make articles such as gloves andoil resistant seals.

Elastomeric articles requiring the highest elongation and greatest easeto stretch, such as surgical or examination gloves, balloons, andcondoms have traditionally been made from natural rubber latex. Whilenitrile rubber products are typically more difficult to stretch, one ofthe advantages of nitrile rubber over natural rubber latex substrates isthat nitrile rubber products do not contain natural latex proteins whichcan become a significant allergy issue for some users. Other advantagesof nitrile materials over natural rubber latex include much betterchemical resistance, especially to fatty and oily substances, and betterpuncture resistance. Hence, nitrile-rubber-based gloves have becomedesirable as a substitute for natural rubber products.

While hospitals, laboratories, or other work environments that may userubber gloves often want to go “latex free” to better protect theirworkers, the normally higher cost of nitrile products often limits theirability to make the change. Another hindrance toward making the changeis that nitrile gloves traditionally have been stiffer, hence are muchless comfortable to wear as compared to similar types of gloves madefrom natural rubber latex materials. For instance, natural rubber latex(NRL) examination gloves typically require a stress of about 2.5 MPa (58psi) to stretch to an elongation of about 300% over original dimensions.This often is referred to as the glove's 300% modulus. Nitrile examgloves, on the other hand, typically require more than twice that amountof stress (˜5 MPa, 116 psi) to achieve the same 300% strain. While vinylcan be another synthetic choice, vinyl is often seen as a lowerperformance choice.

Currently, no synthetic latex examination gloves are available on thecommercial market that exhibit force-strain properties that are close tothat of natural rubber latex gloves, not to mention being either similaror the same as natural rubber-based gloves in these terms. Force-strainproperties refer to a direct measurement of how a material responds(stretches) in response to an applied force, regardless of the thicknessof the material. Stress-strain properties in contrast measure theresponse to an applied force per unit cross sectional area of thematerial.

Nitrile rubber, a synthetic polymer often used in emulsion (latex) formto manufacture medical and industrial gloves is a random terpolymer ofacrylonitrile, butadiene, and a carboxylic acid such as methacrylicacid. It can be crosslinked by two separate mechanisms to improve itsstrength and chemical resistance. The first mechanism of crosslinkingoccurs by ionically bonding carboxylic acid groups together usingmultivalent metal ions. These ions are typically supplied throughaddition of zinc oxide to the emulsion. Normally the strength andstiffness/softness of the polymer is very sensitive to this type ofcrosslinking. The other crosslinking mechanism is a covalentcrosslinking of the butadiene segments of the polymer using sulfur andcatalysts known as rubber accelerators. This covalent crosslinking isespecially important for development of chemical resistance. Gloves areoften formed by first placing a coagulant solution, often calciumnitrate on ceramic glove moulds, then dipping into the nitrile latex tocause local gelation of nitrile rubber over the mould surface.

Several prior approaches to softening nitrile rubber articles involvedstrongly limiting or completely omitting zinc oxide and other materialscapable of ionically crosslinking carboxylated nitrile rubber, such asthose described in U.S. Pat. Nos. 6,031,042 and 6,451,893. In additionto not yielding force-strain properties similar to those of comparablenatural rubber products, this method can result in a material havinglower strength, the need for higher curing temperatures, the need forextraordinarily high levels of other chemicals that may cause skinirritation, or it may lead to processing difficulties such as thickeningof the nitrile latex before dipping.

Other approaches to making a nitrile glove more comfortable, such asthose described in U.S. Pat. Nos. 5,014,362 and 6,566,435, have reliedon stress relaxation over time and require constantly applied levels ofstrain to cause that relaxation or softening. Such determinationmeasures are difficult to maintain and can be unrealistic in real worldpractice and use.

A need exists for a nitrile-based polymer article that can successfullycombine the benefits of nitrile materials with the greater pliability orsoftness of natural rubber latex without the need to apply conditionsrequired for softening caused by stress relaxation. There is a need fora kind of nitrile glove that can incorporate a polymer formulation andproduct dimensions to simulate the comfort and softness associated withnatural rubber latex products, while simultaneously maintaining theprotective and non-allergenic properties of nitrile rubber. The glove,when worn, still enables the elastomeric material to exhibit physicalstrain or stress profiles similar to those of natural rubber, withoutexposure to natural rubber's associated problems.

SUMMARY OF THE INVENTION

The present invention pertains to an elastomeric, nitrile rubber articlethat exhibits the force-strain characterstics of related polyisoprenerubber articles, while preserving the tensile strength and protectiveproperties of conventional nitrile rubber. In particular, the inventiondescribes relatively thin elastic articles, such as glove, that isthinner and more pliable or softer than conventional nitrile gloves, butdesigned still to retain the protective properties and maintainsufficient strength for industrial or laboratory work and all medicalprocedures in which nitrile gloves are normally worn. The thinner“soft-nitrile” glove exhibits force-strain response characteristicssimilar to that of natural (polyisoprene) rubber gloves.

The elastic modulus of the nitrile material ranges from about 3 MPa toabout 6 MPa, and the material can maintain a tensile strength in therange of about 30 or 32 MPa to about 56 or 58 MPa. While this range ofmodulus alone in a glove of standard thickness is not sufficient toyield natural rubber force-strain response, reducing the thickness ofthe article in addition to lowering its modulus meets the desired goal.While conventional nitrile examination gloves have a thickness of about0.14±0.02 mm, the nitrile gloves according to the present invention arethinner, ranging from about 0.05 mm to about 0.10 or 0.11 mm, measuredat the palm area defined by American Society for Testing and Materials(ASTM) standard D-412-98a (Reapproved 2002). For example, the nitrilegloves according to the present invention may have a thickness in thepalm region of less than about 0.12 mm. As another example, the glovesmay have a thickness in the range of from about 0.5 mm to about 0.12 mm.As yet another example, the palm region of the glove may have athickness of about 0.6 mm to about 0.11 mm. As another example, a glovemade according to the present invention may have a palm thickness in arange between about 0.05 and 0.10 mm.

According to the present invention, by simultaneously controlling thelevel of crosslinking materials in the nitrile formulation and theproper article thickness, in which both kinds of controls are selectedto maximize material strength and minimize the force needed to stretchthe material, it is believed that one can yield a material with forceresponse behavior similar to that of a natural latex glove of greaterthickness. The crosslinking of the carboxylic acid groups is controlledby the amount and types of ionic materials added to the nitrile emulsionbefore it is used to produce dipped articles. The thickness of thearticle can be controlled by a variety of means during the dippingprocess.

The present approach permits one to employ more reasonable orstandardized levels of chemicals and process parameters to maximize thestrength potential of the nitrile rubber, while still producing a glovethat is more pliable and more comfortable to wear than conventionalnitrile-made articles. The present approach has advantages over theprior art. The present invention affords good flexibility in the overalllevels of crosslinking agents, and gives good rate of covalentcrosslinking without the need for high temperatures. It allowstraditional amounts of crosslinking agents and accelerators to be usedwithout the complications often resulting in levels of these chemicalsbeing too high or too low. Too low a level of metal oxide, for examplecan result in reduced quality of the gelation process, or can causethickening at elevated pH levels of about 8.5 and greater.

This approach does not rely on the need for stress relaxation over aperiod of time as long as about 10-15 minutes, nor a constant strain toeffect that relaxation as others have described in previous attempts tomake a more comfortable glove. The advantageous force-strain response ofthe present nitrile rubber-based material can be appreciated immediatelyby the user. The new type of nitrile-polymer can be adapted to be morepliable and comfortable to wear.

It is believed that the particular properties of the present“soft-nitrile” materials arise in part from the nature of the nitrilecomposition, which includes about a 50:50 blend of two acrylo-nitrilecompositions. On one hand, the first nitrile composition has softer, orin other words has a lower modulus relative to the second nitrilecomposition. On the other hand, the second composition exhibits betterfilm forming properties than the first composition. The properties ofeach composition helps to make the combined mixture for better dipprocessing as well as a softer more pliable material. The blend of thetwo compositions together achieves a synergistic effect. Such aphenomenon is a rare occurrence in the nitrile art. The orientation orplacement of carboxyl groups on the nitrile polymer molecules—eitheroutside or inside—can affect the reactivity of the carboxyl groups withcations, such as magnesium or zinc.

The present invention also details a cost effective process or means tomanufacture such soft-nitrile gloves. The process entails providing amould, applying a coagulant coating to a mould, covering at least aportion of the mould surface with nitrile compositions, such asdescribed above, curing the nitrile composition to form a substrate, andstripping the nitrile substrate from the mould.

Additional features and advantageous of the present invention will berevealed in the following detailed description. Both the foregoingsummary and the following detailed description and examples are merelyrepresentative of the invention, and are intended to provide an overviewfor understanding the invention as claimed.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a graph showing stress-strain curves, illustrating thedifference in relative amount of stretch deformation caused by a widerange of stresses applied to samples from gloves made from a naturalrubber latex, three conventional nitrile formulations, andchlorinated/non-chlorinated versions of the present inventive nitrileformulation.

FIG. 2 is a graph showing the force-strain relationship for the samesamples.

FIG. 3 is an enlargement of the force-strain graph in FIG. 2, showingthe region between zero and four-hundred percent strain.

FIG. 4 is an illustration of the force required to break samples whentested according to ASTM D-412-98a. Ranges and averages are shown.

DETAILED DESCRIPTION OF THE INVENTION

The present invention describes the creation of elastic articles, suchas gloves, made from a nitrile polymer formulation that exhibit physicalcharacteristics similar to those of comparable natural rubber latexarticles. A desirable attribute for elastomeric articles that are wornon the body is softness or pliability of the polymeric material. Theinvention describes the use of nitrile-based rubber formulations formanufacturing articles that have good physical strength and chemicalresistance while also being softer (i.e., having a lower elasticmodulus) than many previous nitrile rubber compositions. As used herein,the terms “elastic” or “elastomeric’ generally refer to a material that,upon application of a force, is stretchable to an extended, biasedlength. Upon release of the stretching, biasing force, the material willsubstantially recover to near net shape or original dimensions;alternatively at least about 50% of the distorted or extendeddimensions. As used herein, the term “stretch-elongation” refers to theamount or percentage that an elastomeric substrate or membrane isstretched or expanded exceeding its original dimensions. The “percentagedeformation” or “percentage elongation” can be determined according tothe following calculation:Final dimension−Initial dimension/Initial dimension×100Alternatively, the amount of elongation may be described in terms of aratio comparing stretched length to the unstretched length. The amountof recovery (retraction upon relaxation of force), however, is a ratiobased on retraction vs. stretched minus unstretched lengths. This usageis not consistent, but common. As illustration, by way of example only,an elastic material having a relaxed, unstretched length of 10centimeters may be elongated to at least about 13.5 centimeters by theapplication of a stretching or biasing force. Upon release of thestretching or biasing force the elastic material will recover to alength of not more than about 12 centimeters.

Traditionally, two ways have been used to create softer, more pliableelastomeric articles. One way is to make the substrate or membrane wallsof the article thinner. The second way is to reduce the elastic modulusof the elastomeric material. Each of these two approaches has associatedbenefits and disadvantages. For instance, in both gloves and condoms, athinner polymer membrane tends to enable the user to experience greatertactile sensitivity. Also, often the thinner the elastic syntheticpolymer walls become the less amount of force is needed to flex,stretch, or deform the article. Thinness, however, can often beassociated with problems, such as weak tensile strength or tendency torupture under use. A lower elastic or Young's modulus, on the otherhand, allows one to retain a relatively thicker substrate and stillimpart ease of flexibility when worn on the hand. Lowering the modulusof a rubber formulation by reducing level of crosslinking in the polymeroften also results in lower strength or lower chemical resistance.

The force response behavior of current nitrile gloves is normally verydifferent from that of similar natural rubber gloves. When similarforces are applied to both kinds of materials, the amount ofinstantaneous stretch would be much higher for the natural rubber glove.While this difference can be reduced through various approaches, suchas, most typically by reducing or even eliminating the amount of metaloxide crosslinking, reduction of the level of metal oxides to theextreme required to close the relatively large gap in difference betweenthe two kinds of polymers often can irrevocably compromise the strengthof the material body, or adversely affects the manufacturing process bydipping (i.e., slower gelation, slower covalent crosslinking, viscosityincrease, etc.) while still not simulating the degree of force responseassociated with natural rubber very closely.

In the present invention, the extent or amount and types of ioniccrosslinking can be controlled by regulating the content of all ionicmaterials during compounding or formulating of the nitrile latex. Ratherthan trying to go to an extreme high or low level of control, however,we have found a balance in the formulae that can produce sufficientlyhigh tensile strength to lower the thickness of the dipped article,while adjusting the thickness of the material so that it requires lessforce to stretch than current synthetic article. By simultaneouslycontrolling the level of crosslinkage in the material compositionformula and the proper substrate thickness for the article, such thatthey maximize material strength and minimize the amount of force tostretch the material, one can yield a material that exhibits a forceresponse behavior similar to that of a natural rubber latex substrate ofsimilar or greater thickness. The crosslinking of the carboxylic acidgroups is controlled by the amount and type of ionic materials added tothe nitrile emulsion before it us used to produce dipped articles. Thethickness of the article can be controlled by a variety of means duringthe dipping process, such as manipulation of the length of time that themould form dwells in or is covered by the emulsion, temperature, ormechanical rotation or pivoting of the mould after withdraw from thedipping bath.

The gloves made using the current invention are less bulky and morepliable to wear, hence providing greater comfort compared toconventional nitrile gloves, and further can lead to cost savings in themanufacture process and ultimately to the consumer. With a thinnermaterial, the wearer also enjoys greater tactile sensation in the handand finger tips than compared with regular gloves. All of theseadvantages can be realized with no compromise in the strength of theglove.

Most nitrile rubber examination gloves that are currently available onthe market have thickness of about 0.12-0.13 mm or greater. According tothe present invention, we can fabricate gloves that have a lower basisweight than conventional gloves. A glove made according to the currentinvention has a palm thickness in a range between about 0.05 and 0.10mm, without sacrificing the strength characteristics associatedtypically with thicker gloves of higher basis weight. While nitrilegloves made according to the present invention are on average 30-50%thinner than other nitrile examination gloves currently on the market,the inventive gloves are designed to still have sufficient strength towithstand industrial, laboratory, or medical procedures in whichexamination gloves are typically worn. A review of many nitrileexamination gloves currently on the market shows the thickness in thepalm area is about 0.12 mm or greater.

The precise point of measurement is that defined in American Society forTesting and Materials (ASTM) test standard D-412-98a (Reapproved 2002),“Standard Test Methods for Vulcanized Rubber and ThermoplasticElastomers—Tension, published January 2003, the contents of which areincorporated herein by reference. These test methods cover proceduresused to evaluate the tensile (tension) properties of vulcanizedthermoset rubbers and thermoplastic elastomers. The determination oftensile properties start with test pieces taken from a sample materialand includes the preparation of specimens and the testing of thespecimens. Specimens may be in the shape of dumbbells, rings, orstraight pieces of uniform cross-sectional area. Measurements of tensilestress, tensile stress at a given elongation, tensile strength, yieldpoint, and ultimate elongation are made on specimens that have not beenprestressed. Tensile stress, tensile strength, and yield point are basedon the original cross-sectional area of a uniform cross-section of thespecimen. Measurement of tensile set is make after a previouslyunstressed specimen has been extended and allowed to retract by aprescribed procedure.

Section I Composition

Carboxylated nitrile, which is a terpolymer of butadiene, acrylonitrile,and organic acid monomers, has at least two properties that make ituseful for manufacturing elastomeric articles. These two features arehigh strength and impermeability to certain hydrocarbon solvents andoils. Compounding and curing the rubber (which is used in latex formfor, e.g., dipping to provide articles of manufacture such as gloves orcondoms) with other ingredients such as curing agents, accelerators, andactivators is generally performed to optimize these properties. Thelevel of each monomer in the polymer and the level of curing affect thelevels of strength and the chemical resistance in the finished article.Polymers with higher levels of acrylonitrile tend to have betterresistance to aliphatic oils and solvents, but are also stiffer thanpolymers that have lower levels of acrylonitrile. While the chemicalnature of the monomers from which the polymer is made offers some degreeof chemical resistance, when the polymer molecules are chemicallycrosslinked, resistance to chemical swelling, permeation, anddissolution greatly increase.

Crosslinking also increases the strength and elasticity of the rubber.Carboxylated nitrile latexes can be chemically crosslinked in at leasttwo ways: the butadiene subunits can be covalently crosslinked withsulfur/accelerator systems; and the carboxylated (organic acid) sitescan be ionically crosslinked with metal oxides or salts. Sulfurcrosslinks often result in large improvements in oil and chemicalresistance. Ionic crosslinks, resulting from, for example, the additionof zinc oxide to the latex, result in a rubber having high tensilestrength, puncture resistance, and abrasion resistance, as well as highelastic modulus (a measure of the force required to stretch a film ofthe rubber), but poor oil and chemical resistance. Many currentlyavailable rubber formulations generally employ a combination of the twocuring mechanisms. For example, in combination with sulfur andaccelerators, carboxylated nitrile latex manufacturers frequentlyrecommend addition of 1-10 parts of zinc oxide per 100 parts of rubber.

While some have described ways to make softer nitrile gloves, such asdetailed in U.S. Pat. Nos. 6,031,042 or 6,451,893, both of which involveformulations that contain no zinc-oxide, the present invention providesa formulation with zinc oxide, which improves the dipping qualities andcure rates. When zinc oxide is not employed, the curing time required toreach an optimum state of cure can be much longer and the curing may beless efficient. This means that the crosslinks are longer (more sulfuratoms per crosslink) and there may be a higher amount of sulfur thatdoes not crosslink polymer chains. The result can be a less-effectivelycured rubber that has lowered heat resistance and less chemicalresistance. Ionic crosslinking, however, often increases the stiffnessof an article made from the rubber. This is a disadvantage forapplications in which a softer rubber is needed. For example, surgicalgloves made of soft rubbers can provide greater tactile sensitivity forthe wearer, which is desirable to improve the surgeon's “feel” duringoperations and to prevent fatigue of the hands.

A more comfortable nitrile glove that is easier to stretch, i.e. haslower elastic Young's modulus, can be made using a polymer whichcontains less acrylonitrile or by crosslinking the polymer to a lesserdegree. These changes, however, often compromise strength, chemicalresistance, or both, resulting in articles that are unsuitable for manyapplications. Accordingly, a soft rubber having strength and chemicalresistance similar to stiffer rubbers is highly desirable.

The rubber membrane of the present invention is more stretchable; hence,it has been found that persons who normally need to wear a large-sizeglove can use a medium-size version of a glove made from the presentnitrile-based composition, without binding or loss of flexible comfort.Moreover, a thinner rubber membrane enhances tactile sensitivity totemperature and surface textures.

While not intending to be bound by theory, it is believed that thematrix structure and strength of the present inventive articles mayresult from the interaction of all ions present in the system, inparticular, divalent or higher valence cations, with the carboxylic acidcomponents of the polymer matrix. Divalent or multivalent cations, suchas Mg, Ca, Zn, Cu, Ti, Cd, Al, Fe, Co, Cr, Mn, and Pb, can crosslinkwith the carboxyl groups of the ionized carboxylic acids, formingrelatively stable bonds. Of these cation species, Mg, Ca, Zn, Cu, or Cdare more desirable. Preferably, the methylacrylic acid monomers arelocated relatively close to each other in the polymer matrix structure;in such a fashion, that a divalent or multivalent cation can crosslinkwith two or more nearby acid units. The positive charge of the cationcan well balance the negative electrons of the acidic carboxyl groups.It is believed that, absent divalent or multivalent cations, multiplepolymer chains in the nitrile emulsions are not well crosslinkedtogether. Monovalent ions, such as K, Na, or H, which do not havesufficient electron capacity to accommodate a bond with a secondmethylacrylic acid unit, may allow weaker forms of associative bonding.Monovalent salts that increase the pH of the system may also swell thelatex particles, making more carboxylic acid groups accessible to othercrosslinking agents. The positive charge of the cation can well balancethe negative electrons of the acidic carboxyl groups.

In addition to slightly lowering, for example, the zinc oxide level ofthe formulation, it was found that addition high levels of mono-valentions is advantageous to maintaining high strength of the material. Thesemonovalent ions can come from alkali agents used to adjust pH of theformulation, or from other salts that do not destabilize the nitrilelatex. A sulfur and rubber accelerator combination is included toprovide the desired level of chemical resistance to the finishedproduct. In some cases, a single dithiocarbamate accelerator added withsulfur is sufficient; in other cases where higher levels of chemicalresistance were needed, a combination of diphenyl guanidine,zinc-mercaptobenzothiazole, and a dithiocarbamate accelerator with thesulfur provide better results.

The base polymer employed in the present nitrile material is aterpolymer composition containing acrylo-nitrile, butadiene, andcarboxylic acid components. It is believed that the particularadvantageous properties of the present soft nitrile materials is due inpart from the nature and interaction of a blend of acrylo-nitrilecomponents in the composition. The blend includes two—a first and asecond—acrylo-nitrile formulations in a compositional ratio ranging,respectively, from about 60:40 to 40:60. The blend of the componentstogether achieves a synergistic effect that helps to produce a softermore pliable material which also manifests better dip-processingcharacteristics. Such a phenomenon is a rare occurrence in the nitrilematerial art. The orientation or placement of carboxyl groups on thenitrile polymer molecules—either outside or inside—can affect thereactivity of the carboxyl groups with zinc ions; hence, it is believedthat some components exhibit softer, lower modulus properties and somecomponents have good film forming properties.

The acrylonitrile content of the blended or combined composition isbetween about 17 to 45% by weight, desirably about 20-40%, and moredesirably about 20-35. Typically, the acrylonitrile content is betweenabout 22 and 28 percent by weight, the methacrylic acid content is lessthan 10 percent, and the remainder of the polymer is butadiene. Themethacrylic acid content should be less than about 15% by weight,preferably about 10%, with butadiene making up the remainder balance ofthe polymer. The base terpolymer is made through a process of emulsionpolymerization, and can be used while still in emulsion form tomanufacture gloves or other elastomeric articles.

The acrylonitrile polymer formulations that may be employed in thepresent invention can have a glass-transition temperature (T_(g)) in arange of between about −15° C. or −16° C. to about −29° C. or −30° C.,under typical properties. In some embodiments, desirable nitrile polymerformulations, such as PolymerLatex X-1133 or Synthomer 6311 availablefrom PolymerLatex GmbH, and Synthomer Ltd., respectively, have a T_(g)between about −18° C. to about −26° C. More desirable nitrileformulations, such as Nantex 635t, commercially available from NantexIndustry Co., Ltd. (Taiwan, R.O.C.), can have a T_(g) of about −23.4° C.to about −25.5° C. The nitrile formulation can afford higher strengththan other commercially available nitrile polymers.

Reducing the thickness of the substrate membrane or skin of anelastomeric glove normally reduces its strength. To make the inventiveglove thinner while still retain high strength properties, we havedeveloped a nitrile polymer that has higher inherent strength comparedto other nitrile latexes on the market. We optimize this strengthadvantage though our formulation and compounding methods. A relativelyhigh pH value, in a range of about 9-12.5 or 13 is desired to optimizingthe glove strength. Particular desirable pH values are about 10-11.5.The acrylonitrile polymer-containing emulsion can be adjusted to thedesired pH values usually, for example, by addition of potassiumhydroxide or ammonium hydroxide, at a concentration of 5 to 10%.

The nitrile emulsion is compounded, or combined with other chemicalsthat aid in forming the glove and give the gloves sufficient strengthand durability for their intended application. Compounding of thethinner glove is done by combining the following materials. Ageneralized formula for this approach is as follows, with all levelslisted in approximate parts per 100 parts of dry rubber:

TABLE 1 Carboxylated Nitrile Latex 100 dry parts Alkali Hydroxide  0-1.5 Zinc Oxide or other Metal Oxide 0.5-1.5 Sulfur 0.5-1.5 RubberAccelerator 0.5-1.5 Titanium Dioxide 0-5 Color Pigment 0-1Any carboxylated “nitrile,” i.e. nitrile butadiene rubber, available ina latex form that may be suitable for dipping can be used. The formulacan be adjusted in the ranges listed above to compensate for the variedinherent properties of the many nitrile latexes available. Some suitableexamples would be Synthomer 6311 nitrile latex, manufactured bySynthomer Sdn Bhd., or Perbunan N Latex X-1133 manufactured byPolymerLatex GmbH. The titanium dioxide is used only to provide adesired level of whiteness or opaqueness.

In certain embodiments, according to the invention, a commercial nitrilelatex solution as received is about 43.5% total solids content (TSC).The present nitrile emulsion compound can be prepared to have a TSC ofabout 15 or 16-25%. In some desirable embodiments the TSC can be about19-22%. Depending on the strength of the coagulant, the time the gloveformer is allowed to dwell in the latex bath; however, this could varyand still can produce thin gloves. The finished glove has a TSC of 100%,since the substrate should not contain appreciable or significantamounts of water.

It is believed, however, that the nitrile butadiene polymer propertiesdo not come from components of the material, but from the structure ofthe polymer, which in turn, is determined by polymerization conditions.Polymer properties are very much affected by the polymer structure.Molecular structure of polymers can be very complex, with variability inmolecular weight, molecular weight distribution, amount of branching,amount of crosslinking during polymerization, many possible types ofchemical addition for diene monomers, etc. When several monomer typesare combined into a polymer such as in a carboxylated acrylonitrilebutadiene polymer used for glove manufacture, the structure becomes evenmore complex. Overall levels of each monomer type and the sequencing ofthe monomer units also contribute to the properties of the resultingpolymer. When the repeating structure of the monomer units are random,such as in the nitrile rubber used for gloves, the physical propertiesof the polymer have increased influence from the polymer linearity (vs.branching) and molecular weight as compared to the properties of ahomopolymer. This is because the properties expected from a regularrepeating structure of a polymer made only from each single monomerchange once that repeating structure is interrupted or otherwise alteredby addition of other types of monomer units. A high level of anyparticular monomer will likely increase the chance of contributingproperties expected from a homopolymer made from that monomer, due toincreased similarity of the repeating structures.

In carboxylated nitrile rubber used for thin glove manufacture, theacrylonitrile and carboxylic acid, which typically total approximately35% add some plastic like character to the polymer with respect toresilience, permanent set, and stress relaxation. They also prevent aregular cis-1,4 repeating structure that would give polybutadiene itshighest resilience and lowest set/relaxation.

A general description of such a carboxylated nitrile rubber would be along-chain random arrangement of it's three component monomers, withbranching and crosslinking. These branched, random terpolymers areformer into discrete tiny particles that are emulsified in water. Inaddition to the polymer structure, the particle structure also plays apart in the final properties of a glove. Parameters such as particlesize, particle size distribution, level of particle agglomeration,particle density, etc., affect how the product is formed, and also it'seventual properties.

In the present invention, the polymer structure includes a randomterpolymer (as opposed to block or alternating terpolymer) ofacrylonitrile, butadiene, and carboxylic acid. The properties depend onthe average molecular weight, the molecular weight distribution, thelinearity or degree of branching, the gel content (crosslinking duringpolymerization), and the microstructure (which monomer units are next toeach other in short sections of the polymer chain.

The formulation adjustments of the present invention can lower the 300%modulus of a nitrile glove to about 3.5 MPa, but this still results inthe nitrile glove requiring more force to stretch (strain) the materialas compared to a natural rubber latex glove. A relatively low level offorce of up to about 3.5 Newtons (N) can be used to stretch the nitrilearticle to about 400% original dimensions. Desirably about less than orequal to about 2.5 N. For example, the nitrile article (e.g., nitrileglove) may be characterized by a force of up to about 1.5 N to cause a300% elongation (F-300) from initial dimensions.

Since the tensile strength of the gloves (i.e., stress required to breakthe material) from the adjusted formula is substantially higher thanthat of typical natural rubber gloves, reducing the glove thickness incombination with a low modulus can produce a nitrile glove with aforce-strain relationship very similar to that of a NRL glove. Acombination of a lower modulus cure system along with proper selectionof glove thickness has yielded a nitrile elastomeric glove that has thesame force-strain characteristics of natural rubber latex substrates. Inother words, when an identical amount of force is applied to the presentnitrile glove and a natural rubber latex glove, each gloves wouldexhibit a similar amount of stretchiness; hence, the two kinds of gloveswould have similar comfort characteristics when worn.

Section II Strength

While nitrile gloves made according to the present invention are onaverage about 30-40% thinner than gloves made from other nitrilematerial formulations currently available on the market, the inventivegloves are designed to still have sufficient strength to be able towithstand industrial or laboratory work, or for all medical proceduresin which the gloves are normally worn. A review of many nitrileexamination gloves currently on the market shows that the thickness inthe palm area of the gloves is about 0.12 mm or greater. The parametersand measurement protocols are defined in American Society for Testingand Materials (ASTM) test standard D-412-98a. In the present invention,we employed the ASTM protocol with no changes. The testing apparatus weuse is an Instron® tensometer, model 5564, with a static load cell ofcapacity about +/−100N, and a XL extensometer. Other similar kinds ofequipments would also work, as long as the machine meets therequirements of the ASTM standard.

As stated, many nitrile examination gloves that are currently availableon the market have thicknesses of about 0.12 mm or greater. According tothe present invention, we can fabricate nitrile gloves that have a lowerbasis weight than conventional gloves. A glove according to the presentinvention has a palm thickness of in a range between about 0.06 and 0.10mm, without sacrificing strength characteristics that are associatedtypically with thicker gloves of heavier basis-weights. “Strength” asused herein can be described as a function of the amount of forcenecessary to break a sample of prescribed shape and dimensions, such asthose used for ASTM test standard D-412. In testing, an inventive glovewith a thickness of about 0.08-0.10 mm in the palm area has an averageforce-at-break reading of about 8.7-10.2 newtons (N), desirably about9.1-9.85 N, and more desirably about 9.18-9.5 N. Current market gloveshave values ranging from about 6.7 to 14.3 N, with most values between7.5 and 10.5 N.

The nitrile material of the elastic article can have a tensile strengthat break in a range of about 30 MPa to about 55 MPa, desirably about 40MPa. Typically, the amount of elongation at break would be in a range ofabout 550-750%, and more likely about 650%. At about 300%stretch-elongation, the modulus of the nitrile material is in a range ofabout 3 MPa to about 6 MPa, desirably about 4 MPa.

The level of the ionic materials is balanced in the formula to achievethe desired modulus. If the product to be made will be very thin, 0.05mm for example, a higher modulus can be tolerated while the lowthickness of the material will still result in a relatively low forcerequired to stretch the article. In this case, the metal oxide could beused at the higher end of the stated range, together with a low tomoderate level (0-0.5 phr) of alkali hydroxide or other monovalent salt.This would insure that the thinnest products would have sufficientlyhigh tensile strength and force-to-break values.

If the desired article will be at the upper end of the thickness rangediscussed in association with this invention, 0.10-0.12 mm, then lowerlevels of metal oxide, together with moderate to high levels of alkalihydroxide (0.5-1.5 phr) would be chosen. Examples of these more specificformulae are:

TABLE 2 A B C D Material Carboxylated Nitrile Latex 100 100 100 100Ammonium Hydroxide 0.4 0.0 0.0 0.78 Potassium Hydroxide 0.0 1.45 1.0 0.0Zinc Oxide 1.1 0.25 0.5 0.25 Sulfur 1.0 1.0 1.0 1.0 Zinc DiethylDithiocarbamate 1.0 1.0 1.0 1.0 Titanium Dioxide 1.0 1.0 1.0 1.0 ColorPigment 0.2 0.2 0.2 0.2 Resulting Properties 300% Modulus (MPa) 6.2 3.64.7 6.3 Tensile Strength (MPa) 43.1 35.0 50.0 50.3 Force to Break at0.05 mm thick (N) 6.5 5.2 7.5 7.5 Force to Break at 0.10 mm thick (N)13.0 10.4 15.0 15.0 Force to 300% Strain (0.05 mm) (N) 0.9 0.5 0.7 0.9Force to 300% Strain (0.10 mm) (N) 1.8 1.0 1.4 1.8Similar Properties for samples cut from a currently marketed naturalrubber latex glove of typical thickness (0.15 mm), and a currentlymarketed Nitrile examination glove of thickness 0.12 mm, bothmanufactured by Kimberly-Clark Corporation:

TABLE 3 Natural Rubber Nitrile Force to Break (N): 10.1 9.6 Force to300% Strain (N): 0.8 2.2

With any of the example formulas, a thickness higher than 0.05 mm wouldbe needed to match the force to break of Kimberly-Clark's currentnatural latex exam glove. While the amount of metal oxide cross linkingagent in the formula may be adjusted slightly downward to achieve alower modulus, a large adjustment is not necessarily needed nor desiredsince high strength resulting from these crosslinks aids in the articlehaving sufficient strength at lowered thickness. With the force requiredbeing directly proportional to the glove thickness, the thicknessrequired for these formulas to have a force at break of 10.1 N would be0.078, 0.097, 0.067, and 0.067 mm respectively for example formulas A,B, C, and D. The corresponding force required to stretch these materials300% based on these thicknesses would be 1.4, 1.0, 0.9, and 1.2 Newtons.While an examination glove made from formula C at 0.067 mm thicknesswould provide extremely similar properties to those of a typical naturallatex examination glove, it can be seen that all of these formulas canbe used to produce gloves with properties similar to those of naturalrubber gloves.

It is believed that combination of a thinner material substratethickness, the nitrile formulation used, an elevated pH≧8.5 or 9, andthe procedural changes to compounding and dipping contribute toimportant differences between the manufacture of the present glove andthe manufacture of other current nitrile examination gloves.

The accompanying figures are charts comparing gloves made 1) accordingto the present invention, 2) commercially available nitrile-based glovesfrom Kimberly-Clark Safeskin™ and 3) other comparative examples. Thecharts in FIGS. 1 and 2 show ranges of data for force to stretch andforce to break. The present invention differs from other nitrile-basedproducts with reference to the straight force readings that are similarto such parameters of natural rubber gloves. This phenomenon is believedto result from a combination of both modification of modulus andrelative thinness of the present inventive products. Generally, thesoft-nitrile articles have a much lower modulus than that of thecurrent, competitive powder-free nitrile glove products. The synergisticeffect of a reduction in thickness and retention of high tensilestrength qualities is an extra feature to making the present inventionunique.

FIG. 1 is a graph that compares the performance of several examples ofglove membranes made from the present invention, currently availablenitrile rubber based materials, and natural rubber latex. Experimentalexamples according to the present invention are indicated as EXP 1 andEXP 2, while comparative examples are shows as COMP A and COMP B. Astandard nitrile is labeled NITRILE, and the natural rubber latex islabeled LATEX. The graph illustrates that the stresses required tostrain (modulus) gloves made from the inventive formulations are reducedrelative to those of current nitrile-based examination gloves. Toclosely simulate the stress-strain behavior of natural rubber latexexamination gloves, however, the stress level of the inventive materialneeds to be further reduced.

FIG. 2 shows that when one reduces the relative thickness of theinventive nitrile gloves, without compromising their barrier or tensileproperties, the force response (force-strain behavior) of these glovescan closely approximate or simulate that of a natural rubber glove,especially at strain levels up to about 300% or 400%, the usual range ofstrain that might be expected when donning and wearing an examinationglove. When extended up to about 500% elongation, the examples made fromthe present inventive formulations can be stretched with a force of lessthan about 2-3 Newtons (N) to stretch, but the comparative examplesrequire about 4 N or greater. FIG. 3 is an enlarged view of the lowstrain portion of the force strain chart up to 400% elongation to moreclearly illustrate the advantageous feature. That is, the force response(also commonly referred to as the force-strain behavior orforce-to-strain response) of these inventive nitrile rubber glovesidentified as EXP 1 and EXP 2 in FIG. 3 closely approximate or simulatethe substantially linear force-to-strain response from initialdimensions of the comparative natural rubber glove, and especially atstrain levels up to about 300% over initial dimensions, the usual rangeof strain that might be expected when donning and wearing an examinationglove.

The thickness of the inventive nitrile gloves were about 0.07-0.10 mm,desirably about 0.08 mm, at the palm region. The thickness of thenatural rubber samples were about 0.15 mm, and the thickness of thecomparative nitrile gloves from Kimberly-Clark and two othermanufacturers were 0.12-0.13 mm.

To illustrate the unique properties of the present invention, data fromtwo different soft-nitrile experimental products is included in thefigures, the difference being assessment with (EXP 1) and without (EXP2) chlorination. The non-chlorinated glove would have properties typicalof a powdered glove or a polymer coated glove. Chlorination and coatingsare standard methods to eliminate the need for powder on gloves. Thelabels NR and nitrile refer to PFE (natural rubber latex) and PFN(nitrile latex) gloves currently manufactured by Kimberly-Clark. Forcomparison sake, two other competitive nitrile gloves, ComparativeExamples A and B are included.

FIG. 4 shows the force required to break the samples that are shown inthe earlier charts. Ranges and average values are shown. As explainedpreviously, the force required to break the samples made using thecurrent invention can be adjusted through the level of ingredients inthe formula or by adjusting the thickness of the glove. Typically,membranes made of natural rubber polymers have a failure point at about10 N. In contrast, force at break values from various iterations of thepresent inventive formulation can be as high as 15 N or more, whilemaintaining the glove thickness less than about 0.10 mm.

Other advantages of the present invention, relative to traditionalnitrile or soft-nitrile formulations include, for example, the abilityto create a thinner substrate than other nitrile exam gloves, whilepreserving the high strength characteristics and chemical resistancecomparable to that of thicker gloves. The thinness can enhance tactilesensitivity, increase comfort (thin glove with low modulus). Cost issuescan also be improved for the benefit of the user. A thinner glove canhelp lower costs in manufacturing because of the relatively lessmaterial needed compared to thicker gloves. Additionally, for instance,one can package about 150 gloves in a standard dispenser instead of 100,for less packaging waste.

An existing problem is a tendency for medical tape to adhere too tightlyto some types of medical gloves that do not have a non-stick coating ontheir exterior, gripping surfaces. This can be a significant problem forexamination gloves that are used with various procedures in hospital andclinical settings. The excessive adhesion often results in damage to theeither glove, tape, or patient (e.g., pull out of IVs, catheters, etc.,when practitioners remove their hands from the area of treatment).Hence, according to another embodiment, a treatment applied to theexterior surface of the gloves can reduce tack of medical tape togloves, gloves to each other (i.e., bricking), or aid in applicationswhere double donning is desirable or required. The tape adhesion problemcan be resolves with an application of a relatively small amount ofmaterial that acts as a release agent. Examples of materials that havebeen successfully evaluated include silicone emulsions and glycerin.

The present nitrile-based gloves are chlorinated and dried usingindustry standard processes and equipment used for treating examinationgloves. The article can have on an exterior surface that is coateddirectly on the naked rubber with a material having a release ornon-adhesive, or anti-adhesive properties, such as but not limited to: asilicone emulsion or silicone oil, a glycerin solution, a combinedmixture of silicone and glycerin/glycerol, a fluorocarbon release agent,or natural or synthetic hydrocarbon-based oils in amounts that do notdegrade the substrate rubber material.

There are many, many types of silicone based materials that would workto lessen medical tape sticking on the nitrile material. For instance,the simplest classifications would be polydimethylsiloxanes ororganofunctional siloxanes. The latter group covers a wide variety ofmaterials including silicone copolymers and terpolymers, amino-modifiedsilicones, alkyl modified and aryl modified silicones, epoxy modified,alkoxy modified, alkenyl modified, silanol modified, carboxy modifiedsilicones, silicone polyethers, and many others. Any silicone materialthat could be expected to have lubricating or release properties wouldwork, such as poly(methylphenyl-siloxane) or dimethicone.

Other types of materials that could be used are any materials that havelubricant or release properties. These would include natural andsynthetic hydrocarbon oils, such as vegetable oils, hydrogenatedpolyolefins, or esters, and fluorocarbons release agents, such aspolytetrifluoroethylene (TEFLON®), or its modified derivatives. If thesematerials are not water soluble (most aren't) they can be used inemulsion form, or applied directly via transfer coating, i.e. spraying,then distributing more evenly by gloves rubbing together duringtumbling, or by contact with a cloth containing the lubricant materialduring tumbling, etc. Vegetable oils are actually related toglycerine—they are sometimes defined as esters of glycerine. Theadvantage of using glycerine is that it is very water soluble, which insome processes make it easier to use. Silicone materials on the otherhand, would have more permanence if the glove requires any rinsing stepsafter application, or if the glove is washed or used in wetapplications.

Although, silicone lubricants are used in surgical glove applications onthe inner (donning) surface to aid in the donning of gloves with damphands, the release agent is applied to the grip-side surfaces of thegloves, preferably, within a rotating chamber, such as modified clothesdrier. The gloves are tumbled and sprayed intermittently with a mixtureof a silicone emulsion or oil and water, such as DC Emulsion 36commercially available from Dow-Corning. We chose a simple material, theDow Corning 36 Emulsion as a cost effective silicone material. Otherpossible grades to include as examples would be “Dow Corning 365, 35%Dimethicone NF Emulsion,” Sil-Wet L-7602” from GE Silicones, or GESilicone Emulsion SM-2164. The coating is allowed to dray betweensprayings. Add-on levels of the release agent can be as low asapproximately 30 or 35 mg up to about 500 mg per glove article, which issufficient to achieve less tacky release of tape from the glove.Preferably, the amount of lubricant is contained on the low end of thisrange, between about 30 to 100 mg, more preferably about 35 to 60 mg.Higher levels may be desired and are sometimes effective, but may causeappearance and residue problems such as streaking.

One can achieve both good peal and tape release properties and aestheticappearance on nitrile gloves treated with a glycerin solution preparedin a concentration of about 3-8%, desirably about 4-5% or 6%.Approximately 6 grams total of each concentration solution is sprayedonto the outside (grip) side of gloves. On one hand, gloves sprayed witha prepared solution at concentration of about 1% or 2% still adheredstrongly to medical tape. On another hand, gloves sprayed with about a10% concentration solution exhibited easy tape release, but had excessglycerol on them, which produced a shiny surface that is typicallyundesired. A combined mixture of silicone and glycerin, would consistmostly of glycerin with a minority percentage of silicone emulsion oroil. For example, on a 6-7 g glove one may apply about 80-100 mg ofglycerin and about 15-30 mg of silicone emulsion or oil. Hence, theratio of glycerin to silicone can be about 6:1 to about 3:1.

Section III Processing

We have found that the order in which the chemicals reagents areincorporated can be significant. One can achieve improved processing andphysical properties in the nitrile material from the order of additionand amounts of reagent materials. The strength of nitrile examinationgloves is typically achieved by ionically crosslinking the organic acidgroups contained within the polymer structure. These chemical groups caninteract with a variety of cations in the system. Some cations arealready in the nitrile emulsion as we receive it—counterions for theanionic surfactant used to manufacture the emulsion and cations from pHadjustment done by the manufacturer to insure stability of the productduring shipment. Other cations are introduced into the system viamaterials that we add to the nitrile emulsion—zinc ions from zinc oxide,and potassium or ammonium ions from further pH adjustment.

Adding a base, such as either potassium or ammonium hydroxide, last inthe compounding sequence increases the strength of the thin materialsubstrate by allowing zinc from the zinc oxide to react more thoroughlywith the acid groups of the nitrile polymer before the level of othercations in the system are greatly increased during the pH adjustment.This procedure also results in a glove that is easier to stretch, asmeasured by force readings at prescribed amounts of stretch or bymodulus, which is the force per unit cross sectional area at specifiedlevels of stretch.

Table 4, provide a summary and comparison of certain physical propertiesfor a number of gloves samples. In particular, the properties includethe force (Newtons) to stretch the elastic glove skin to about 400%original dimensions, the force (Newtons) to stretch to rupture of theglove, and the relative thickness of each sample. The physical valuesfor a typical natural rubber latex gloves is given as a control.Examples 1-12 represent acrylo-nitrile butadiene gloves samplesaccording to the present invention. Comparative Examples 1-8 representtypical samples of commercially-available nitrile-based gloves.

TABLE 4 GLOVE FORCE (N) at FORCE (N) THICKNESS EXAMPLE No. 400%EXTENSION TO BREAK (mm) Natural Rubber 1.3 10.1  0.155 Latex (NRL)control Example 1 1.5 9.18 0.08 Example 2 1.7 9.22 0.09 Example 3 2.29.60 0.07 Example 4 1.9 9.28 0.08 Example 5 2.0 9.32 0.08 Example 6 1.49.15 0.06 Example 7 2.1 11.25 0.12 Example 8 1.6 9.20 0.08 Example 9 2.510.48 0.11 Example 10 1.8 9.25 0.09 Example 11 1.6 9.19 0.08 Example 121.5 9.20 0.07 Comp. Example 1 6.0 8.2 0.12 (powder free nitrile) Comp.Example 2 6.8 8.9 0.13 Comp. Example 3 6.6 9.5 0.15 Comp. Example 4 7.67.55 0.14 Comp. Example 5 8.0 10.3 0.12 Comp. Example 6 5.3 14.3 0.12Comp. Example 7 8.4 13.5 0.12 Comp. Example 8 4.2 6.7 0.13As one can observe, for the examples according to the present invention,one need apply only a fraction (e.g., about ½ to about ¼) of the amountof force at 400% elongation, required to achieve the same level ofstretching in the eight comparative examples. This suggests that thepresent invention achieves a more soft and supple elastic skin, moreclosely resembling the stretch exhibited by natural rubber latex.

The invention is useful in manufacturing process for elastomericarticles composed of acrylonitrile materials. The invention affords, theability to produce nitrile-based articles which closely mimics thephysical properties of elastomeric articles made from natural rubberlatex. The invention can be advantageously incorporated into themanufacture of a variety of products, such as medical examination orsurgical gloves, condoms, probe covers, dental dams, finger cots,catheters, and the like.

The present invention has been described both in general and in detailby way of examples. Persons skilled in the art will understand that theinvention is not limited necessarily to the specific embodimentsdisclosed. Modifications and variations may be made without departingfrom the scope of the invention as defined by the following claims ortheir equivalents, including equivalent components presently known, orto be developed, which may be used within the scope of the presentinvention. Hence, unless changes otherwise depart from the scope of theinvention, the changes should be construed as being included herein.

A. Glove Formation

In the manufacturing of nitrile gloves, the solids content of thenitrile emulsion is reduced from 40-45% to approximately 23% in order tocontrol the thickness of the glove. To further reduce glove thickness,solids content is reduced to approximately 20%. The thinner glove of thepresent invention can be fabricated by means of a coagulant dip-coatingprocess. The process or method entails: providing a clean glove form ormould that is preheated to approximately 55-60° C., preferably about 58°C. The prepared mould is dipped into an aqueous solution of calciumnitrate. The mould, with coagulant on its surface, are dried andreheated to approximately 70° C.±5° C., and dipped into a bath of acompounded nitrile emulsion, forming a gelled glove. A beads can berolled at the top of the glove cuff. The mould with gelled glovesubstrate is soaked in water to remove all of the water-soluble materialcomponents. A mould with gelled glove is dried in ovens at a temperaturein the range of about 80° C. to below about 100° C. When the mould withgelled glove substrate is then heated to a higher temperature, thesulfur reacts with other chemicals and crosslinks the methylacrylic acidunits in the nitrile polymer. Afterwards, the glove is removed from themould, and the glove surfaces are treated with chlorinated water toreduce tack. Finally, the resulting gloves are dried and readied forpackaging.

A faster entry and exit speeds of the glove mould into the nitrileemulsion dip can provide a more even thickness profile to the glove, anddue to reduced difference in residence time of the fingertip and cuffareas of the moulds in the compounded nitrile emulsion. The mould can beextracted from the dip bath at or near an initial vertical position andraised such that the finger tips are elevated to a horizontal or greaterthan horizontal position (e.g, tilted to an angle about 20° to 45° abovehorizontal) for a brief period of a few seconds up to about 40 seconds.Quickly following, lower the finger tips to a position or angle betweenhorizontal and initial vertical, while rolling the mould along itslongitudinal axis. The raising and lowering action can be repeated in asinusoidal or wave-like motion. This process can enable the nitrile todistribute more evenly over the former and produce a thinner overallsubstrate product.

Another feature for the present thinner glove is the ability to havechemical resistance equal or better than that of current, thicker,nitrile examination gloves. This could be accomplished using acombination of vulcanization accelerators. The combination includes adithiocarbamate, a thiazole, and a guanidine compound, which, desirably,are present in the composition according to a ratio of about 1:1:2,respectively. In particular, according to an embodiment, the compoundsare diphenyl guanidine (DPG), zinc mercaptobenzothizole (ZMBT), andzincdiethyldithiocarbamate (ZDEC), at about 0.5 phr DPG, 0.25 phr ZMBT,0.25 phr ZDEC.

This combination of accelerators is very similar to that described inU.S. Pat. No. 6,828,387, incorporated herein, although the level ofthese chemicals is reduced by approximately 50%. This prior patent isfor curing (vulcanizing) polyisoprene rubber. Unlike the '387 patent, itis believed that a double crosslinking process is involved in thepresent invention. In other words, relative to polyisoprene materials,crosslinking is accomplished by means of a covalent double bond in theisoprene molecule, with the present nitrile-based system, thecrosslinkage is a covalent interaction with respect to the butadienecomponent and an ionic interaction with respect to the zinc ion and thecarboxyl group of the methyl acrylic acid.

The invention is useful in manufacturing process for elastomericarticles composed of acrylonitrile materials. The invention affords, theability to produce nitrile-based articles which closely mimics thephysical properties of elastomeric articles made from natural rubberlatex, without the associated latex-protein, allergenic-response issues.The invention can be advantageously incorporated into the manufacture ofa variety of products, such as medical examination or surgical gloves,condoms, probe covers, dental dams, finger cots, catheters, and thelike.

The present invention has been described both in general and in detailby way of examples. Persons skilled in the art will understand that theinvention is not limited necessarily to the specific embodimentsdisclosed. Modifications and variations may be made without departingfrom the scope of the invention as defined by the following claims ortheir equivalents, including equivalent components presently known, orto be developed, which may be used within the scope of the presentinvention. Hence, unless changes otherwise depart from the scope of theinvention, the changes should be construed as being included herein.

1. An elastomeric glove comprising: a glove substrate formed from aformulation containing a carboxylated nitrile-butadiene randomterpolymer composition having a glass transition temperature (Tg) in arange of about −15° C. to about −30° C., and between 0-1.5 parts of analkali monovalent salt and 0.5-1.5 parts of a metallic oxide wherein thetotal combined amount of the alkali monovalent salt and metallic oxideis greater than 1.0 part per 100 dry parts of rubber consisting of thecarboxylated nitrile-butadiene random terpolymer, the glove substratehaving a thickness of from about 0.05 to about 0.12 mm in a palm regionof the glove, the glove substrate requiring a force of less than orequal to about 2.5 N to cause a 400 percent elongation (F-400) frominitial dimensions and the glove substrate further exhibiting asubstantially linear force-to-strain response up to about 300 percentelongation (F-300) from initial dimensions when tested in accordancewith ASTM D412-98a and the glove further comprising an inner, donningsurface and an exterior, gripping surface, and a release coating on theexterior, gripping surface, wherein the release coating is selected froma silicone emulsion, silicone oil, a glycerin solution, a mixture ofsilicone and glycerin/glycerol, a fluorocarbon release agent, a naturalor synthetic hydrocarbon-based oil, and combinations thereof.
 2. Theelastomeric glove according to claim 1, wherein said terpolymercomposition comprises an acrylonitrile content of between about 17 wt. %to 45 wt. %, a carboxylic acid content of up to about 15 wt. %, and theremaining balance of the terpolymer being butadiene.
 3. The elastomericglove of claim 1, wherein the force at 300% elongation (F-300) of theglove substrate is up to about 1.5 N.
 4. The elastomeric glove accordingto claim 1, wherein the glove may be extended up to about 500%elongation with a force of less than 3 N when tested in accordance withASTM D412-98a.
 5. The elastomeric glove according to claim 1, whereinthe release coating is a mixture of silicone and glycerin wherein theratio of silicone and glycerin is about 1:3 to about 1:6.
 6. Theelastomeric glove according to claim 1, wherein the release coating isan organofunctional siloxane present in an amount of approximately 30 mgup to about 500 mg per glove.
 7. The elastomeric glove according toclaim 6, wherein the release coating is an organofunctional siloxanepresent in an amount between about 30 to 100 mg.
 8. The elastomericglove according to claim 1, wherein said nitrile glove has a thicknessas measured at the palm region that is in the range from about 0.05 mmto about 0.10 mm.
 9. The elastomeric glove according to claim 1, whereinsaid glove substrate has an elongation at break of about 550 to 750%.10. An elastomeric glove comprising: a glove substrate formed from aformulation containing a carboxylated nitrile-butadiene randomterpolymer composition having an acrylonitrile content of between about17 wt. % to 45 wt. %, a carboxylic acid content of up to about 15 wt. %,and the remaining balance of the terpolymer being butadiene, and a glasstransition temperature (Tg) in a range of about −15° C. to about −30°C., and between 0-1.5 parts of an alkali monovalent salt and 0.5-1.5parts of a metallic oxide wherein the total combined amount of thealkali monovalent salt and metallic oxide is greater than 1.0 part per100 dry parts of rubber consisting of the carboxylated nitrile-butadienerandom terpolymer, the glove substrate having a thickness in a palmregion of from about 0.05 to about 0.12 mm, and exhibiting aforce-strain profile of from about 0.8 N to about 2.0 N, over anextension range of about 100-400%, an elastic modulus in a range of fromabout 3 MPa to about 6 MPa at about 300% stretch elongation, and a forceto break of between about 8-14 Newtons (N) when tested in accordancewith ASTM D412-98a, and the glove further comprising an inner, donningsurface and an exterior, gripping surface, and a release coating on theexterior, gripping surface, wherein the release coating is selected froma silicone emulsion, silicone oil, a glycerin solution, a mixture ofsilicone and glycerin/glycerol, a fluorocarbon release agent, a naturalor synthetic hydrocarbon-based oil, and combinations thereof.
 11. Theelastomeric glove according to claim 10, wherein said palm region has athickness in the range of about 0.06 mm to about 0.11 mm.
 12. Theelastomeric glove according to claim 10, wherein said glove substrateexhibits a tensile strength of about 35 MPa to about 55 MPa.
 13. Theelastomeric glove according to claim 10, wherein said glove substrateexhibits a tensile strength of about 40 to 50 MPa.
 14. The elastomericglove according to claim 10, wherein said glove substrate has anelongation at break in a range of about 550-750%.
 15. The elastomericglove according to claim 10, wherein said force-at-break is about 9-11N.
 16. An elastomeric glove comprising: a glove substrate with a palmregion having a thickness in the range from about 0.05 mm to about 0.12mm; the glove substrate being formed from a formulation containing acarboxylated nitrile-butadiene random terpolymer composition having anacrylonitrile content of between about 17 wt. % to 45 wt. %, acarboxylic acid content of up to about 15 wt. %, and the remainingbalance of the terpolymer being butadiene, and a glass transitiontemperature (Tg) in a range of about −15° C. to about −30° C., andbetween 0-1.5 parts of an alkali monovalent salt and 0.5-1.5 parts of ametallic oxide wherein the total combined amount of the alkalimonovalent salt and metallic oxide is greater than 1.0 part per 100 dryparts of rubber consisting of the carboxylated nitrile-butadiene randomterpolymer, in which the metallic oxide provides ionic cross linking andthe alkali monovalent salt can provide associative bonding; the glovesubstrate requiring a force of less than or equal to about 2.5 N tocause a 400 percent elongation (F-400) from initial dimensions and inwhich the glove substrate exhibits a force-to-break of between about8-14 N and a substantially linear force-to-strain response up to about300 percent elongation from initial dimensions which occurs at a up toabout 1.5 N when tested in accordance with ASTM D412-98a, and the glovefurther comprising an inner, donning surface and an exterior, grippingsurface, and a release coating on the exterior, gripping surface,wherein the release coating is selected from a silicone emulsion,silicone oil, a glycerin solution, a mixture of silicone andglycerin/glycerol, a fluorocarbon release agent, a natural or synthetichydrocarbon-based oil, and combinations thereof.
 17. An elastomericnitrile-rubber glove comprising: a glove substrate formed fromformulation containing a carboxylated nitrile-butadiene randomterpolymer composition having a glass transition temperature (Tg) in arange of about −15° C. to about −30° C., and in terms of parts per 100dry parts of rubber consisting of carboxylated nitrile-butadiene randomterpolymer, 0-1.5 parts of an alkali monovalent salt and 0.5-1.5 partsof a metallic oxide wherein the total combined amount of the alkalimonovalent salt and metallic oxide is greater than 1.0 part per 100 dryparts of rubber consisting of the carboxylated nitrile-butadiene randomterpolymer, the glove substrate having a thickness, as measured in apalm region of said nitrile-rubber glove, that is from about 0.05 toabout 0.12 mm; and which exhibits a force-to-strain profile of fromabout 0.8 N to about 2.0 N, over an extension range of about 100-400%,an elastic modulus in a range of from about 3 MPa to about 6 MPa atabout 300% stretch elongation, and the glove substrate exhibits a forceto break from about 6.5 N to about 15 N when tested in accordance withASTM D412-98a, and the glove further comprising an inner, donningsurface and an exterior, gripping surface, and a release coating on theexterior, gripping surface, wherein the release coating is selected froma silicone emulsion, silicone oil, a glycerin solution, a mixture ofsilicone and glycerin/glycerol, a fluorocarbon release agent, a naturalor synthetic hydrocarbon-based oil, and combinations thereof.
 18. Theelastomeric nitrile-rubber glove according to claim 17, wherein theglove substrate further exhibits a substantially linear force-to-strainresponse up to about 300 percent elongation (F-300) from initialdimensions at a force up to about 1.5 N when tested in accordance withASTM D412-98a.
 19. The elastomeric nitrile-rubber glove according toclaim 17, wherein the glove substrate may be extended up to about 500%elongation with a force of less than 3 N.
 20. The elastomeric nitrilerubber glove according to claim 17, wherein the glove substrate exhibitsa tensile strength of about 30 MPa to about 55 MPa.
 21. The elastomericnitrile-rubber glove according to claim 17, wherein the glove substratehas an elastic modulus about 4 MPa at about 300% stretch elongation.